TrkA, TrkB and TrkC, which make up the Trk receptor family, are high affinity receptor tyrosine kinases activated by a group of soluble growth factors called neurotrophins (NT) (Curr Opin Neurobiol, 2001, 11, 272-280).
Inhibitors of the Trk/neurotrophin pathway have been demonstrated to be effective in numerous animal models of pain. For example, sustained blockade of neurotrophin receptors TrkA, TrkB and TrkC reduces non-malignant skeletal pain (Bone, 2011, 48(2), 389-398). Administration of NGF receptor (TrkA) inhibitor K252a showed significant suppression of mechanical hyperalgesia (relevant to the pathogenesis of myofascial pain syndrome (MPS)) in animal models (J. Pain, Article in Press, 2011, 12(10), 1059-1068). Antagonistic NGF and TrkA antibodies have been shown to be efficacious in inflammatory and neuropathic pain animal models (Neuroscience, 1994, 62, 327-331; J. Pain, 2004, 5, 157-163; Nat. Med., 1995, 1, 774-780; Pain, 2005, 116, 8-16; Pain, 2003, 105, 489-497) and neuropathic pain animal models (Eur. J. Neurosci., 1999, 11, 837-846; Pain, 1999, 79, 265-274; Pain, 1999, 81, 245-255; Neurosci. Lett., 2003, 336, 117-120).
NGF secreted by tumor cells and tumor invading macrophages has been shown to directly stimulate TrkA located on peripheral pain fibers. It has also been demonstrated in various tumor models in both mice and rats that neutralizing NGF with a monoclonal antibody inhibits cancer related pain. Further, activation of the BDNF/TrkB pathway has been implicated in numerous studies as a modulator of various types of pain including inflammatory pain (J. Physiol. 2005, 569:685-95), neuropathic pain (Proc. Natl. Acad. Sci. USA 1999, 96:7714-18) and surgical pain (Molecular Pain, 2008, 4(28), 1-11). Since TrkA kinase has been demonstrated to serve as a mediator of NGF driven biological responses, inhibitors of TrkA and/or other Trk kinases may provide an effective treatment for various pain conditions.
Inhibition of the neurotrophin/Trk pathway with NGF antibodies or non-selective small molecule inhibitors of Trk A, B and C has been shown to be effective in treatment of pre-clinical models of inflammatory diseases such as asthma (Pharmacol. Therapeut., 2008, 117(1), 52-76), interstitial cystitis (J. Urology, 2005, 173(3), 1016-21), inflammatory bowel diseases including ulcerative colitis and Crohn's disease (Gut, 2000, 46(5), 670-678) and inflammatory skin diseases such as atopic dermatitis (Arc Dermatol Res., 2006, 298(1), 31-37), eczema and psoriasis (J. Investig Dermatol., 2004, 122(3), 812-819).
The current treatment regimes for pain conditions utilize several classes of compounds. The opiates apart from being potentially addictive have several adverse effects such as emesis, constipation, dose-related respiratory depression. Nonsteroidal anti-inflammatory analgesics (NSAID) also have drawbacks such as gastric ulceration, dyspepsia and insufficient efficacy in treating severe pain. Accordingly, there is a continuing need for new and more effective treatments for the relief of pain, especially chronic pain. Several classes of small molecule inhibitors of Trk kinases said to be useful for treating pain or cancer are known (Expert Opin. Ther. Patents, 2009, 19(3), 305-319).
U.S. Publication No. 20110195948 describes substituted pyrazolo[1,5-a]pyrimidine compounds as Trk kinase inhibitors.
JP Publication No. 2003231687 describes a series of pyrazolyl condensed cyclic compounds as Trk inhibitors.
PCT Publication No. 200505427 describes compounds containing a 1,4,5,6-tetrahydropyrrolo[3,4-c]pyrazole bicyclic scaffold as TrkA inhibitors.
PCT Publication No. 2004011461 describes a series of isothiazole derivatives as Trk inhibitors.